We have been investigating the properties of a group of proteins called Tau antigens that can be isolated from various lines of SV40 -transformed cells by immunoprecipitation with serum obtained from hasmters bearing SV40 - induced tumors (anti-T serum). Work in several laboratories has shown that these proteins can be isolated from mouse embryonal carcinoma cells and from cells transformed by various retroviruses, papovaviruses, and chemicals. They have not been detected in significant amounts in normal, untransformed cells or in tumor antigen-negative revertant cell lines derived from SV40 - transformed mouse or rat cells. We have determined the relationship among Tau antigens isolated from various lines of SV40 - transformed cells by comparing their methionine - containing tryptic peptides by 2 - dimensional fingerprinting. All Tau antigens studied so far appear to share some common peptides. Those isolated from closely related speices (i.e. mouse and rat) show the greatest similarity. Within a certain species (mouse, hamster), the Tau antigens are very closely related or perhaps identical. Recently, we have detected Tau antigen in monkey cells infected with SV40. This protein is closely related to the Tau antigen isolated from a line of human cells transformed by SV40. Finally, we have compared the Tau antigens isolated from hamster cells transformed by different primate papovaviruses. In this study, we showed that the peptide fingerprints of the protein isolated from cell transformed by SV40, BK and JC viruses were indistinguishable.